test(h2): isolate connection churn seeds

[mcollina]

Scope the HTTP/2 churn test cleanup to each seed so its TLS server, sessions, agent, and keepalive timer are released before the next seed starts.

Native Linux x64 investigation showed that the remaining Node.js 24 failures are process-level memory-corruption crashes, not assertions from this test. Although the cores usually fail later in V8's concurrent Maglev compiler, the source is Node's HTTP/2 binding calling nghttp2 send/close paths reentrantly while nghttp2_session_mem_recv() is active.

Skip this stress test on Node.js 24 until the HTTP/2 fix in nodejs/node#65093 reaches a release. Other Node.js versions continue to run both seeded churn cases.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (3926499) to head (fa9e682).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5643      +/-   ##
==========================================
+ Coverage   93.40%   93.44%   +0.04%     
==========================================
  Files         110      110              
  Lines       38695    38776      +81     
==========================================
+ Hits        36144    36236      +92     
+ Misses       2551     2540      -11     

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[metcoder95]

tests doesn’t seem happy

[mcollina]

It’s worse than that, there is a bug in Node 24 that’s causing a sigsev.